As a container used for filling beverages, for example, a container such as a bottle made of glass has been widely used.
Recently, the bottle mentioned above is to be lightened in weight in order to benefit convenience in transportation thereof. A means of forming a coating layer on the surface of the bottle is applied in order to prevent the bottle from weakening in strength due to the introduction of lightening the bottle in weight. Metal oxide coating layer such as SnO2 or TiO2 is formed by means of a so-called hot end coating process on the surface of the bottle for this purpose.
The hot end coating process is a technique in which a reaction gas is sprayed on the surface of the bottle, during the time when the surface thereof still has a relatively higher temperature prior to the application of a slow cooling thereto upon manufacturing the bottle, to form an oxide coating layer of SnO2 or TiO2 on the surface of the bottle.
The thickness of the coating layer should fall within a prescribed scope to maintain a mechanical strength (i.e., durability) of the bottle. With a thin coating layer outside the scope, bottles contact with each other upon transporting thereof to result in causing a scratch on the surface of the bottle so that it becomes difficult to maintain the desired strength of the bottle, thus cracking the bottle.
On the other hand, with a thick coating layer outside the scope, a rainbow-like pattern is produced on the surface of the bottle due to the interference of the light (especially, in a visible light range), which pattern looks like a oil film adhering on the surface of the bottle, to result in deteriorating a fine view, lowering the value of the commodity and wasting the reaction gas.
Accordingly, it is desirable to severely inspect the thickness of the coating layer formed on the surface of the bottle.
Conventionally, the inspection of the thickness of the coating layer formed on the surface of the bottle has been carried out by an apparatus for measuring the thickness thereof by means of contacting the bottle to be inspected (hereinafter referred to as a "contact-type measuring apparatus"), for example, a hot end coating meter manufactured by American Glass Research, Inc. Thereby the thickness of the coating layer is detected and measured by contacting a light emitting/sensoring device with the surface of the bottle.
However, in the contact-type measuring apparatus, it is necessary to adjust an angle of the light emitting/sensoring device so as to receive the maximum amount of the light. There is therefore a problem that it requires a well trained skill to measure the thickness of the coating layer, and furthermore, even a man of well trained skill has to take time to do the same.
Furthermore, in the contact-type measuring apparatus, silicone oil as an optical coupling fluid has to be immersed on the surface of the bottle when the light emitting/sensoring device is contacted with the bottle. It is therefore necessary to wipe out the immersion fluid after the thickness is measured. Because it takes time to wipe out the painted oil, the bottles used for measuring the thickness of the coating layer are discarded in general.
In general, the bottles are sampled to detect and measure the thickness of the coating layer in the manufacturing process of the bottles. It is therefore necessary to increase the number of the inspections in order to obtain the overall information about the situation of the coating layer of the manufactured bottles, thus increasing the number of bottles to be discarded, and lowering efficiency of the inspection.
Japanese Patent Provisional Publication No. 131,547/91 discloses a bottle to be recycled and repeatedly used with a coating layer formed on the surface thereof (which has a SnO2 coating layer of about 100 nm thickness)(hereinafter referred to as a "returnable bottle") so as to lighten the weight of the bottle for beverages like beer and increase the number of repetitive use.
However, the above-mentioned conventional contact-type measuring apparatus has a problem that it can measure only the thickness of the coating layer up to about 60 nm, it is impossible to detect and measure the thickness of the coating layer of about 100 nm mentioned above.
In addition, a bottle with a prescribed thickness of a coating layer formed on the surface thereof can be recycled and used, thus increasing the number of the repetitive use of the bottle. However, since the returnable bottle collected from the market is in general repetitively washed in a washing machine with the use of a heated alkaline solution (for example, 4% of a caustic soda aqueous solution at the temperature of 80.degree. C.), the coating layer on the surface of the bottle may be deteriorated by the heated alkaline solution.
Due to the deterioration of the coating layer, the surface of the bottle looks like a whitish, thus deteriorating a fine external view of the bottle to lower the value of the commodity as a bottled product, even if the bottle has a sufficient strength to be recycled and used.
More specifically, the coating layer, for example, the SnO2 coating layer has pinholes from 2 to 3 .mu.m in diameter therein from the beginning, as illustrated in FIG. 13(a). Then, the coating layer is repeatedly washed in the heated alkaline solution so that the pinholes gradually grow deeper and larger up to about 10 .mu.m in diameter, as illustrated in FIG. 13(b).
When the pinholes grow so large as mentioned above, a light passing through the bottle is scattered by the pinholes in the coating layer. Furthermore, when the number of the pinholes increase, the color of the coating layer turns gradually from an original pale gold to silver, thus lowering its transparency.
When the number of the pinholes increase more, the surface of the bottle becomes like a ground glass so that the overall or part of the surface of the bottle lose a shine, resulting in looking like a whitish. The bottles with remarkably deteriorated coating layer on the surface thereof have to be discarded even if they have a sufficient strength to be recycled.
The separation of the bottles with a deteriorated coating layer from those with a good coating layer has been carried out by an inspector by means of the observation with his eyes.
However, there is a problem in the detection of the deteriorated coating layer by means of the observation with the eyes that it is difficult to establish an objective index to identify the extent of the deterioration. Thus, the accuracy of the inspection has to be left to the subjective judgement of the respective inspectors themselves to cause a different result in the respective judgements. In addition, there is a problem that the observation with the eyes by the inspector inclines to causing an oversight or limits the inspecting speed.
The present invention is made to solve the above mentioned problems in the inspection of the thickness or deteriorating situation of the coating layer formed on the surface of the bottle.
More specifically, the purpose of the present invention is to provide an apparatus and method for inspecting a coating layer, in which a thickness of the coating layer formed on the surface of the bottle can be measured without contacting the bottle, the inspection of the bottles can be carried out not to the selected sampling bottles but to all the bottles, and even a coating layer having a thick thickness can be measured.
Another purpose of the invention is to provide an apparatus and method for inspecting a coating layer, in which the deteriorating situation of the coating layer formed on the surface of the bottle can be automatically operated and differentiated on the basis of an objective standard, and a high inspecting speed is possible.